In order to reduce the emission amount of carbon dioxide, an engine is proposed which includes an idling stop mechanism of an integrated starter generator (ISG) configured to stop the engine when the vehicle stops, and to start the engine quickly when the accelerator pedal is pushed so that the vehicle starts to move.
FIGS. 7A and 7B illustrate a belt transmission device of an engine including an idling stop mechanism of an ISG capable of driving engine accessories and starting the engine. The belt transmission device further includes a crankshaft pulley P1 attached to a crankshaft 51, a starter generator pulley P2 attached to the rotary shaft of the starter generator 52 of the ISG, an engine accessory pulley P3 attached to the rotary shaft of an engine accessory 53 such as a water pump, and a belt 54 trained around the pulleys P1, P2 and P3. While the engine is running normally, as illustrated in FIG. 7A, the crankshaft pulley P1 rotates in the direction of the shown arrow so as to drive the starter generator 52 and the engine accessory 53, thereby enabling the starter generator 52 to function as a generator.
On the other hand, when the engine is started by driving the starter generator 52, as illustrated in FIG. 7B, the starter generator pulley P2 rotates in the direction of the shown arrow so as to rotate the crankshaft pulley P1, thereby enabling the starter generator 52 to function as a starter.
Such a belt transmission device as described above further includes a tension pulley 55 mounted to the belt portion 54a of the belt 54 located between the crankshaft pulley P1 and the starter generator pulley P2, a pivotable pulley arm 56 rotatably supporting the tension pulley 55, and a hydraulic auto-tensioner A, by applying an adjustment force to the pulley arm 56, biasing the pulley arm 56 in the direction in which the tension pulley 55 presses the belt 54, thereby absorbing the tension fluctuations of the belt 54.
Japanese Unexamined Patent Application Publication No. 2009-275757 (JP 2009-275757) discloses such a hydraulic auto-tensioner as the hydraulic auto-tensioner A. The hydraulic auto-tensioner of JP 2009-275757 includes a cylinder, a valve sleeve standing on the bottom surface of the cylinder, a rod having a lower end portion slidably inserted in the valve sleeve such that a pressure chamber is defined in the valve sleeve, a spring seat provided on the top end of the rod, and a return spring mounted between the spring seat and the bottom surface of the cylinder so as to bias the rod and the valve sleeve in the direction in which the rod protrudes from the valve sleeve.
Also, in the hydraulic auto-tensioner of JP 2009-275757, a sealed reservoir chamber is defined between the inner periphery of the cylinder and the outer periphery of the valve sleeve, an oil passage is formed in the bottom of the cylinder such that the lower portion of the pressure chamber communicates with the lower portion of the reservoir chamber through the oil passage, and a check valve is provided in the bottom end of the valve sleeve. The check valve is configured such that when a pushing force is applied to the rod so that the pressure in the pressure chamber exceeds the pressure in the reservoir chamber, the check valve is closed so as to block the communication between the pressure chamber and the oil passage.
The hydraulic auto-tensioner described above is configured such that a coupling piece provided on the top surface of the spring seat is rotatably coupled to the engine block E illustrated in FIG. 7A. A coupling piece provided on the bottom surface of the cylinder is coupled to the pulley arm 56, and the check valve is closed when a pushing force is applied to the rod from the belt 54 through the tension pulley 55 and the pulley arm 56. Oil in the pressure chamber flows through a leakage gap defined between the sliding surfaces of the valve sleeve and the rod, the viscous resistance of the oil flowing in the leakage gap generates a hydraulic damper force in the pressure chamber, and this hydraulic damper force dampens the above pushing force.
Since the above conventional hydraulic auto-tensioner is configured such that when a pushing force is applied to the rod, oil in the pressure chamber leaks through the single leakage gap, defined between the sliding surfaces of the valve sleeve and the rod, it is impossible to apply an appropriate tension to the belt 54 both while the engine is running normally and when the engine is started by driving the starter generator 52.
Namely, if the size of the leakage gap is set such that the tension fluctuations of the belt 54 can be absorbed while the engine is running normally, the leakage gap becomes large. As a result thereof, when the engine is started by driving the starter generator 52, the rod is pushed into the pressure chamber to a large degree, so that the belt 54 loosens, thereby generating slippage at the contact portions of the belt 54 and each pulley P1, P2, P3. This may shorten the service life of the belt, and cause the engine to malfunction when started by the starter generator 52.
On the other hand, if the size of the leakage gap is set such that the tension fluctuations of the belt 54 can be absorbed when the engine is started by driving the starter generator 52, the leakage gap becomes small. As a result thereof, while the engine is running normally, the tension of the belt 54 becomes too high or extremely strong. This is likely to damage the belt 54 or the bearing rotatably supporting each pulley P1, P2, P3, thus increasing fuel consumption.
It is an object of the present invention to provide a hydraulic auto-tensioner capable of applying an appropriate tension to the belt both while the engine is running normally and when the engine is started by driving the starter generator.